In a chromatograph apparatus, a set of data representing a chromatogram with the horizontal axis indicating time and the vertical axis indicating the signal strength (e.g. output voltage) are obtained by analyzing a sample (such data are hereinafter called the “chromatogram data”). A data processing system for a chromatograph detects a peak appearing on such a chromatogram, identifies a substance corresponding to that peak from the peak position (retention time) with reference to a previously set identification table, and calculates the concentration and/or quantity of the substance from the height and/or area of the peak.
Such a data processing system normally has limits on the magnitude of the signal that can be processed, due to hardware limitations of the signal-processing circuits including an A/D converter. The system cannot correctly perform calculations if the magnitude of the input signal is higher than the upper limit or lower than the lower limit.
Besides such a limitation related to signal processing, there is a problem that the reliability of the detection result obtained with a detector of a chromatograph apparatus varies with the signal level of the detector. For example, in a device used as a detector for a liquid chromatograph (such as an ultraviolet-visible spectrophotometer or photodiode array detector), if the concentration of the component in the sample is too low, the accuracy of the quantitative determination deteriorates due to the influence of noise in the detection signal. Conversely, if the component concentration is too high, the non-linearity of the detection signal becomes noticeable, so that the accuracy of the quantitative determination similarly deteriorates. Accordingly, when an analysis is performed with a conventional chromatograph apparatus, the sample needs to be appropriately diluted so that the component concentrations in the sample will be included in a predetermined range (dynamic range).